Some great online plant identification guides can be found at the Go Botany, Dave's Garden and U.S. Department of Agriculture's Plants Database sites. Each of these sites offers extensive information to help users identify and catalog plants.
Go Botany's simple plant identification key offers an easy-to-use interface that allows users to classify plants by moving from general to specific characteristics. Users begin by identifying a plant's main group and then work down through more specific subgroups. Go Botany features high-resolution photos to aid in identification and also offers informative videos about different plant types.
Dave's Garden is a gardening site that offers PlantFiles, which claims to be the largest plant database in the world. With information on over 200,000 different plants and a collection of over 360,000 photos, the guide is certainly extensive. PlantFiles allows users to search for plants by common name, botanical name or characteristics. With nearly 60,000 contributors, the site's database is one of the most comprehensive on the web.
The U.S. Department of Agriculture hosts the Plants Database, a collection of standardized information on the mosses, liverworts, hornworts, lichens and vascular plants found throughout the United States. With photos, distribution data, maps, species information and more, the site is a useful tool for anyone looking to identify American plants. The site's U.S.-oriented focus makes it less useful for international users, but it's a great resource for those in the United States.